1. Field of the Invention
The present invention is generally related to electronic circuits, and more particularly related to techniques for compensating the frequency drift of a reference frequency due to the change of temperature.
2. Description of the Related Art
To ensure all electronic parts to work properly or in synchronization, providing an accurate timing clock signal is very important in electronic circuit designs. Usually, such a timing clock signal is produced in a crystal oscillator that is an electronic circuit using the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a timing clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters/receivers. One of the factors that causes a timing clock signal different from the design is the temperature that may affect the piezoelectric material and operation of the crystal oscillator. As the temperature changes, the frequency from the crystal oscillator also changes. In reality, electronic devices, such as portable computers, portable phones and electronic meters, may be used in a wide variety of environments in which temperature varies, it is important that these devices operate as designed without failures or malfunctions due to the changes of the temperature.
Many modern communication devices, such as the GPS and GSM systems, require a highly accurate and stable frequency to increase the sensitivity of a transceiver therein and to reduce the acquisition/track time. Unfortunately, the output of a crystal oscillator tends to drift with ages and temperature changes. A simple crystal oscillator (XO) does not provide a means for controlling the crystal's frequency variation as the ambient temperature changes. Due to the stringent requirements, it is not possible to use a cost-effective stand-alone crystal oscillator in a cellular system without some frequency tuning support from the base station.
The high-stability frequency sources in the wireless communication device or mobile handset generally include a digitally controlled crystal oscillator (DCXO) or temperature-compensated crystal oscillator (TCXO). Unfortunately, the DCXO circuit requires large capacitors to perform frequency corrections on the crystal oscillator. Thus, it is extremely expensive to use the DCXO circuit, especially for a deep submicron CMOS process. Moreover, the switching of a large number of capacitors in the DCXO to adjust the crystal oscillator frequency may result in frequency beating effects that exhibit themselves as spurs in the generated frequency output. Furthermore, the DCXO circuit requires a digitally-controlled compensated frequency signal to compensate the frequency drift when temperature changes.
A simple design is to save a plurality of frequency compensation values, which are obtained by testing at corresponding test temperature points with a fixed temperature interval, in a look-up table. When the frequency is required to be compensated at a certain temperature, the frequency compensation value corresponding to the certain temperature is obtained from the look-up table to compensate the frequency drift. At these temperature points, there are obvious frequency jumps which may influence the functionality of the system. One possible solution is to get the proper frequency compensation value by interpolation of two or more adjacent frequency compensation values saved in the look-up table, but this may complicate the hardware design and degrade the compensation precision of the frequency. Another possible solution is to decrease the interval and increase the number of the frequency compensation values saved in the look-up table, but this may increase the memory and increase test temperature points. Thus, it is useful to find an efficient way to generate the frequency compensation value while using minimal hardware cost.
In a conventional temperature-compensated crystal oscillator (TCXO), a thermostat generates a correction voltage to keep the oscillator's frequency constant. Such a voltage-controlled TCXO has a temperature sensor that generates a linear voltage in proportion to the temperature. With a 3rd-order linear function voltage generator and a voltage controlled crystal oscillator circuit (VCXO), the outputs of the temperature sensor and the 3rd-order function voltage generator are provided to the VCXO which compensates for the temperature vs. frequency characteristics of the crystal being used.
However, such a voltage-controlled TCXO first requires a high quality crystal to meet the 3rd-order linear compensation requirement, which is expensive, particularly, when the size of the crystal is reduced. It is also difficult to achieve high frequency stability and accuracy, as the maximal output frequency of a crystal oscillator is limited. Further, it is difficult to control small frequency changes (e.g. less than 1.0 Hz) in a voltage-controlled TCXO as it is difficult to generate accurately an analog voltage in the range of micro volts.
Therefore, there is a need for a low-cost, low-noise, low-memory and high accurate solution for generating a frequency in a wide frequency range and to be temperature compensated over a wide temperature range.